Among the most difficult steps in fabricating certain types of semiconductor devices such as micromechanical devices, microelectromechanical devices or microfluidic devices is the removal of an oxide sacrificial material which is used to build up the structure of the devices and which is then removed to complete the fabrication of the devices. The sacrificial material can be used, for example, to separate and precisely space layers or elements within the semiconductor device; can be patterned to form a mandrel or mold to define the shape of certain elements of the semiconductor device formed from deposited structural layers (e.g. polysilicon, silicon nitride, or metals); or can be used as a covering to protect certain elements (e.g. bond pads) during further processing steps.
The removal of the oxide sacrificial material generally requires the use of a selective etchant such as hydrofluoric acid (HF) in liquid or vapor form to dissolve away the oxide sacrificial material over a period of up to several hours or more. A high degree of selectivity is required by the etchant so that other layers in the device which are generally formed from polysilicon (i.e. polycrystalline silicon), silicon nitride, or metals are not substantially chemically attacked and damaged. Further information about the etching of oxide sacrificial materials can be found in an article by J. Buhler et al entitled “Silicon Dioxide Sacrificial Layer Etching in Surface Micromachining,” (Journal of Micromechanics and Microengineering, vol. 7, pp; R1-R13, 1997).
Conventional etchants comprising HF at concentrations of about 50% by weight or less are generally unsuitable for etching oxide sacrificial materials in the presence of an exposed metal layer comprising aluminum since the aluminum is chemically attacked at an unacceptably high rate by hydronium ions present in the HF etchant. A carboxylic acid etching solution, as disclosed in U.S. Pat. No. 5,824,601 to Dao, has been developed to improve the etch selectivity to metals including aluminum. However, the HF content in this carboxylic acid etching solution is low (about 8%) so that the time required for removal of an oxide sacrificial material is excessive for many applications.
The present invention provides an etch composition and method that overcomes the difficulties of the prior art and provides a relatively high etch rate for an oxide sacrificial material while minimizing etching of other materials within a semiconductor device that comprise polysilicon, silicon nitride or metals.
The present invention further provides an etch composition and method that is compatible with high-purity semiconductor manufacturing operations.
These and other advantages of the present invention will become evident to those skilled in the art.